Power cables may be used for both direct current (DC) or alternating current (AC) transmission or distribution.
Cables for power transmission or distribution at medium voltage (MV) or high voltage (HV) generally are provided with a cable core comprising a metallic electric conductor (usually aluminium or copper) surrounded by—from the radially innermost layer to the radially outermost layer—an inner semiconductive layer and an insulating layer. The cable core is further surrounded by an outer semiconductive layer, a metal screen (usually aluminium or copper) and an outer sheath.
In the present description, the term “medium voltage” is used to refer to a voltage typically from about 1 kV to about 30 kV and the term “high voltage” refers to a voltage above 30 kV.
Aluminium has the advantage of being lighter and cheaper than copper, but it is prone to oxidation and corrosion in the presence of water/moisture, thus cables comprising aluminium elements should be endowed with a moisture or water barrier to prevent water penetrating and reaching the aluminium element/s.
As a moisture or water barrier, a longitudinally sealed sheet of metal or plastic/metal laminate can be provided around the core/s. The metal can be aluminium. In the case of a laminate, the plastic layer is generally positioned facing towards the outer jacket of the cable and in contact thereto.
Diverse problems can arise due to water contacting the cable core, and then the penetration of water into cables, and stagnation therein, is an event that should be avoided. After manufacturing, cables are usually stored and shipped with protection caps on their heads.
However, the penetration and stagnation of water within the cable core can occur despite the above precautions. In particular, water penetration and stagnation cannot be excluded during installation, for example due to negligence of the installing personnel, but also through any defect in the polymeric sheath of the power cable which exposes the cable aluminium parts to the environment.
Components involved in the corrosion issue depend on cable type. A non exhaustive list can include: screen wires, equalizing tape, water barrier, sheets, etc.
In the presence of water, aluminium is known to form a protective oxide film stable within a wide pH range, from about 4 to 8 (see, e.g., Aluminium Corrosion, UK Aluminium Industry Fact Sheet 2 by ALFED, www.alfed.org.uk).
As reported by Khaled et al., “The inhibitive effect of some tetrazole derivatives towards Al corrosion in acid solution: Chemical, electrochemical and theoretical studies”, Materials Chemistry and Physics, 113, 2009, pp. 150-158, aluminium protective oxide film is amphoteric and dissolves substantially when the metal is exposed to high concentrations of acids or bases. Under these circumstances, corrosion inhibitors should be used because the solubility of the oxide film increases above and below pH range of from 4 to 8 and aluminium exhibits uniform attack. Inhibitors are used to prevent metal dissolution.
Khaled et al. relates to corrosion inhibition of aluminium in a solution of hydrochloric acid 1.0M in the absence and presence of different concentrations of tetrazole derivatives namely, 1-phenyl-1H-tetrazole-5-thiol (A), 1-phenyl-1H-tetrazole (B), 1H-tetrazol-5-amine (C), 1H-tetrazole (D). The reduction in the dissolution of aluminium in the presence of these tested compounds was attributed to the sulphur atom present in the thio group found in compound A as well as to the amino group and the heterocyclic rings.
Tetrazole compounds have been reported in other literature and patent references to have inhibiting action on the corrosion of other non-ferrous metals, such as silver and copper.
Among them, F. Zucchi et al., “Tetrazole Derivatives as Corrosion Inhibitors for Copper in Chloride Solutions”, Corrosion Science, Vol. 38, No. 11, pp. 2019-2029, 1996 relates to the inhibiting action of some tetrazole derivatives on the copper corrosion in chloride solutions. Among the tested derivatives, 5-phenyl-tetrazole (5Ph-T) and 5-mercapto-1-phenyl-tetrazole (5Mc-1 Ph-T) decrease their inhibiting ability at 80° C. The polarisation conductance data demonstrate that only 5Mc-1 Ph-T is able to maintain its protective characteristics for almost 60′ at 40° C.
U.S. Pat. No. 5,744,069 relates to a water soluble metal anticorrosive agent comprising certain tetrazole compounds for non-ferrous metals such as copper, copper alloys and super-hard alloys. The water soluble metal anticorrosive agent comprising a tetrazole compound is represented by the following formula (A):

wherein R1 and R2 each indicate hydrogen, an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group, a phenyl group, an alkylphenyl group, an amino group, a mercapto group or an alkylmercapto group.
U.S. Pat. No. 4,873,139 relates to a technique for imparting corrosion resistance to silver and copper surfaces by contacting such surfaces with 1-phenyl-1H-tetrazole-5-thiol.